The invention concerns a method of drying wood in a wood-working or construction material industry, and a system therefor.
A conventional method of drying construction materials under vacuum generally comprises the steps of blowing down the materials by a heat carrier, subjecting them to vacuum blowing, and sequentially repeating the previous steps at multiple times. The materials are adiabatically held during each cycle between heating and vacuum blowing. The duration of the adiabatic holding equals that of the blowing down. The blowing down is performed by a steam-and-gas mixture with a temperature of 160xc2x0 C. and a moisture content of 15 g/kg of the material for 6 minutes. The vacuum blowing is carried out at the residual pressure of 300 mm Hg [1].
Another conventional method of drying wood comprises the steps of subjecting the wood to a hot air blast with simultaneously removing the vapor evolved therefrom, subjecting it to vacuum blowing, and cooling it off. The wood is preliminarily held in vacuum of 10 kPa, subjected to the hot air blast at a temperature of 50 to 60xc2x0 C. with simultaneously removing the evolved vapor firstly at the air rate of 2.75 under 30 kPa, and secondly at the air rate of 3.25 m/s under 20 kPa. Then, the cooling-off is carried out by air with a temperature of 4-20xc2x0 C. in a vacuum of firstly 15 kPa and then 10 kPa at the air rate of 3.54 m/sec and 45 m/sec, respectively.[2]
Another conventional method of drying wood in the drying chamber comprises the steps of subjecting wood to blast of heated air with removal of the evolved vapors, subjecting it to vacuum blowing, and cooling it off in atmospheric conditions. The duration for subjecting it to the blast of heated air and the vacuum blowing amounts 45 to 120 seconds.[3]
Another conventional method of drying wood in the drying chamber is to blow down the wood by a heat carrier with removal of the evolved vapors. This method comprises the steps of heating wood up to the volume average temperature of 80 to 100xc2x0 C., subjecting it to vacuum blowing, and blowing it down by a heat carrier. When the vacuum blowing is held until the pressure in the pressure-tight chamber reaches the atmosphere, the wood is dried up to 30% of the moisture content. Then, the vacuum blowing is carried out at the residual pressure of 10 to 50 mmHg for 30 to 120 minutes. After the vacuum blowing, the condensation is drained, and then, blow-down is conducted by a heat carrier with a temperature of 80 to 150xc2x0 C. for the time equal to that of the vacuum blowing. Thus, the process of drying to 30% of the moisture is repeated until the total time reaches the value, defined by a ratio:       τ    1    =                    P        ycl            xc3x97              (                              W            h                    -          30                )                    1.57      xc3x97              S                  1          ⁢                      m3            2                                    d          ·          m                    xc3x97      V      xc3x97      100      
Where
Pyclxe2x80x94conditional density of designated material, kg/m3
Whxe2x80x94initial moisture of designated material, %
30xe2x80x94final moisture of designated material at the stage of removal of free moisture, %
1.57xe2x80x94amount of moisture, evolved from 1 m2 of conditional timber after 1 hr at vacuum blowing, kg/m2 h   S      1    ⁢          m3      2            d    ·    m  
Thereafter, the process of drying up to the final moisture is repeated until the total time of the vacuum blowing reaches the value defined by the following ratio:       τ    2    =                    P        ycl            xc3x97              (                  30          -                      W            h                          )                    1.57      xc3x97              S                  1          ⁢                      m3            2                                    d          ·          m                    xc3x97      V      xc3x97      100      
Wkxe2x80x94final moisture of desiccated material, % [4].
Thus, such conventional methods suffer increased consumption of the time, power, cost, etc. taken for obtaining the final product.
It is an object of the present invention to provide a method of drying wood which reduces the time, power, and cost taken for obtaining the finally dried wood product.
According to an aspect of the present invention, a method of drying timbers loaded in a drying chamber comprises the steps of heating the drying chamber by a heating system up to a temperature of 80 to 100xc2x0 C., subjecting the timbers to vacuum blowing by connecting the inside of the drying chamber with a vacuum chamber (receiver) evacuated by a rotary pump until the inside pressure of the drying chamber drops to 1 to 10 mmHg, disconnecting the inside of the drying chamber from the vacuum chamber, connecting the drying chamber with the atmosphere. When the inside of the drying chamber is connected with the vacuum chamber, the moisture content of the timbers is sharply reduced, so that their temperature sharply drops. Thereafter, when it is disconnected from the vacuum chamber, and connected with the atmosphere, the inside temperature of the drying chamber is again increased. Meanwhile, the heating system is worked during the whole process. The above steps constitute one cycle that is sequentially repeated until the moisture content of the timbers drops to a desired level. Preferably, the connection between the drying chamber with the vacuum chamber is made of 0.1 to 0.5 seconds. Pressure release is made in the vacuum for 0.5 to 5.0 seconds until reaching equilibrium moisture. The blow-down in the vacuum is carried out by a heat carrier with a temperature of 80 to 150xc2x0 C. until reaching the volume average material temperature of 80 to 90xc2x0 C. with the subsequent pressure release in the drying chamber lower than the equilibrium, thus providing high-speed vacuum blowing.
According to another aspect of the present invention, a system of drying timbers loaded in a drying chamber comprises a heating system for heating the drying chamber up to a temperature of 80 to 100xc2x0 C., a vacuum chamber connected with the inside of the drying chamber for subjecting the timbers to vacuum blowing, a vacuum pump connected with the vacuum chamber, a manifold for connecting the drying chamber with the vacuum pump, a condenser connected with the vacuum chamber for draining the condensate therein, a first automatic quick-action valve for connecting the drying chamber with the atmosphere, and a second automatic quick-action valve for connecting the condenser with the vacuum chamber. Preferably, the heating system is designed so as to blow a hot air tangentially (vertically) to the timbers. The drying chamber is divided into a plurality of zones each having a quick reacting vacuum valve to the heating system. The inside of the drying chamber is equipped with a plurality of elements for equivalently supplying the hot air along the height of the timbers.
In this case, the diameter of a connecting manifold between the drying chamber and the vacuum chamber may be calculated by the following formula:   d  =                              (                      P            +                          P              0                                )                ⁢        128        ⁢                  xe2x80x83                ⁢        η        ⁢                  xe2x80x83                ⁢        l        ⁢                  xe2x80x83                ⁢                  V          0                                      P          ·                      P            0                          ⁢        t        ⁢                  xe2x80x83                ⁢        π              4  
Wherein
Pxe2x80x94pressure in the drying chamber
Poxe2x80x94pressure in the vacuum chamber
xcex7xe2x80x94kinetic viscosity, cSt
lxe2x80x94length of the manifold from the drying chamber to the vacuum chamber
Voxe2x80x94working free volume of the drying chamber
txe2x80x94time of evacuating lower than the equilibrium